Microelectromechanical systems (MEMS) technology is for manufacturing devices using a technique of microfabrication of semiconductor material, such as silicon. The MEMS technology facilitates mass production, downsizing, and cost-reduction of devices, as well as integration of the devices with integrated circuits (ICs). The MEMS technology is widely used in manufacturing sensors, actuators, and filters, such as acceleration sensors, angular acceleration sensors, inkjet printer heads, and high frequency filters.
Mechanical characteristics, such as dimensions and an elastic modulus, of the devices manufactured from a semiconductor material by the MEMS technology vary due to a change in ambient temperatures. A resonance frequency of a MEMS resonator, a device manufactured by the MEMS technology, depends on its dimensions (a thermal expansion coefficient) and an elastic modulus of the material. Accordingly, a change in the oscillating frequency of the MEMS resonator due to the temperature change is much larger than that of a crystal resonator, and frequency temperature characteristics that indicate the change in the oscillating frequency to the temperature change is not very favorable. For example, in an oscillation mode in which the frequency is determined based on a length, the frequency of the oscillation of the MEMS resonator decreases when the material expands as the temperature rises. In addition, when the material becomes softer due to the decrease of the elastic modulus along with the temperature rise, the frequency of the oscillation decreases. For example, a frequency of a silicon resonator, a MEMS resonator, changes by +30 ppm as the temperature changes by 1° C. Thus, an MEMS device is not easily used as a reference oscillator unit that generates a high frequency wave used for clocks for determining operational timing, broadcasting, and communication.
While mechanical characteristics of a crystal resonator made from crystals also change according to the temperature, it is possible to provide superior frequency temperature characteristics by changing its cut angle as the crystals are an anisotropic single crystal material. Thus, crystal resonators are used as reference resonators in various electronic devices.
However, crystal resonators can hardly have a small size or a low profile. due to a processing method and a shape of the crystal resonators. Crystal resonators cannot have their cost reduced since complicated adjustment steps are required. Even though ICs have a small size, the size of a receiver is not reduced due to the large size of crystal resonators.
In order to use an MEMS resonator as a reference resonator, the change of a frequency due to the temperature is compensated. FIG. 33 is a block diagram of conventional synthesizer 201 described in Patent Document 1. Synthesizer 201 compensates the change of an oscillation frequency due to a temperature of reference oscillation unit 202. Synthesizer 201 includes voltage controlled oscillator (VCO) 206 capable of changing an oscillation frequency, frequency divider 207 that frequency-divides an oscillation signal output from the VCO, frequency divider 203 that frequency-divides an oscillation signal output from reference oscillation unit 202, comparator 204 that outputs a signal according to a phase difference between the signals divided by frequency dividers 203 and 207, and lowpass filter (LPF) 205 that integrates the signal output from comparator 204. LPF 205 integrates the signal output from comparator 204, and converts the signal into a voltage having a frequency close to that of a direct current. Based on this voltage, VCO 206 changes the frequency of the oscillation signal and outputs as a local oscillation signal. Temperature sensor 208 detects an ambient temperature of reference oscillation unit 202. Control circuit 211 determines a dividing ratio for frequency divider 207 based on a channel to be received and the detected temperature. Temperature sensor 208 outputs an analog signal based on the detected temperature. This analog signal is converted into a digital signal by analog/digital (A/D) converter 209. Nonvolatile memory 210 previously stores plural correction values respectively corresponding to plural temperatures, and reads one of the plural correction values based on the digital signal output from A/D converter 209 and transmits the read value to control circuit 211. Control circuit 211 determines the frequency dividing ratio of frequency divider 207 based on the transmitted correction value.
In synthesizer 201, detection accuracy of temperature sensor 208 is on the order of ±0.1° C. Thus, reference oscillation unit 202 either carries out oscillation using a resonator, such as a crystal resonator, having a changing rate of the frequency to the temperature is small, or is used in a system that does not require favorable frequency temperature characteristics.    Patent Document 1: JP03-209917A